Conventional magnetic superconductors: coexistence of singlet superconductivity and magnetic order

Abstract

The basic physics of bulk magnetic superconductors (MS), related to the problem of the coexistence of singlet superconductivity (SC) and magnetic order, is reviewed. The interplay between the exchange (EX) and electromagnetic (EM) interaction is discussed. It is argued that the singlet SC and uniform ferromagnetic (F) order practically never coexist. In the case of their mutual coexistence the F order is modified into a domain-like or spiral structure depending on magnetic anisotropy. It turns out that this situation occurs in several superconductors such as ErRh 4B 4, HoMo 6S 8, HoMo 6Se 8 with electronic and in AuIn 2 with nuclear magnetic order. The latter problem is briefly discussed. The coexistence of SC with antiferromagnetism (AF) is more favorable than with the modified F order. Very interesting physics is realized in AF systems with SC and weak-ferromagnetism which results in an very rich phase diagram. A number of properties of magnetic superconductors in magnetic field are very peculiar, especially near the (ferro)magnetic transition temperature, where the upper critical field becomes smaller than the thermodynamical critical field. The interesting physics of Josephson junctions based on MS with spiral magnetic order is also discussed. The existence of the triplet pairing amplitude F ↑ ↑ ( F ↓ ↓ ) in MS with rotating magnetization (the effect recently rediscovered in SFS junctions) gives rise to the so called π-contact. Furthermore, the interplay of the superconducting and magnetic phase in such a contact renders possible a new type of coupled Josephson-qubits in a single Josephson junction. To cite this article: M.L. Kulić, C. R. Physique 7 (2006).